Transcript bb - StangBio

Evolution
Ch 22-26
AP Biology 2012
Triassic
Permian
225
Seed Plants
Land Plants
Birds
Mammals
Reptiles
Insects
Amphibians
Teleost Fish
Jawless Fish
Chordates
Molluscs
Flowering Plants
180
Dinosaurs
Jurassic
135
Arthropods
Cretaceous
63
Multicellular Animals
Tertiary
Green Algae
1.5
Photosynthetic Bacteria
Quaternary
Anaerobic Bacteria
mya
280
Carboniferous
350
Devonian
Silurian
Ordovician
Cambrian
Ediacaran
400
430
500
570
700
Precambrian,
Proterozoic,
&
Archarozoic 4500
Life’s Natural History is a record of Successions & Extinctions
LaMarck
 Organisms adapted to
their environments by
acquiring traits

change in their life time
 Disuse
organisms lost parts because they did not use them — like
the missing eyes & digestive system of the tapeworm
 Perfection with Use & Need
the constant use of an organ leads that organ to increase
in size — like the muscles of a blacksmith or the large ears
of a night-flying bat

transmit acquired characteristics to next
generation
Charles Darwin
 1809-1882
 British naturalist
 Proposed the idea of
evolution by natural
selection
 Collected data on voyage of
Beagle, plus compared to
more than 500 other studies
& experiments
Voyage of the HMS Beagle
 Invited to travel around the world
1831-1836 (22 years old!)
 makes many observations of nature

 main mission of the Beagle was to chart
South American coastline
Robert Fitzroy
Voyage of the HMS Beagle
 Stopped in Galapagos Islands

500 miles off coast of Ecuador
Succession of types
Armadillos are native to the
Americas, with most species
found in South America.
Why should extinct
armadillo-like species
& living armadillos be
found on the same
continent?
Glyptodont fossils are also
unique to South America.
Mylodon (left) Giant
ground sloth (extinct)
“This wonderful relationship
in the same continent between
the dead and the living will…throw more light
on the appearance of organic beings on our earth,
and their disappearance from it,
than any other class of facts.”
Unique species
So Many Questions!
 Why…





…does a 2 mile forested island tend to have more unique
species than a 2 mile inland forest?
…are fish on the east coast of Panama more closely related to
Bahamian fish, than to fish on the west coast of Panama?
…do desert animals on different continents tend to have
similar body features, yet are more closely related to nearby
non-desert animals than to each other?
…do fossil species closer to the surface tend to be more
similar to current species than deeper fossils?
…do species tend to have features that match their
environment?
 Yet all species have ill-fitting body structures, none are “perfect?”
Evolution
 Biological evolution is the change in a
population’s genetic makeup over
generations.
Occurs by more than one mechanism
 The first (accurate) proposed
mechanism was Darwinian natural
selection

Natural Selection
 When several prerequisites or conditions are in place:
 Variation
 Inheritance
 Competition
 …the inevitable result is differential reproductive
success = some segments of the population have
more offspring on average than others.
Variation
 Variation - Simply put, everyone isn’t identical!

Specifically, this is referring to phenotypic variation - variation
in expressed traits.
 In a given environment at a given time, a variation may be
helpful to the individual that bears it, harmful, or neither.

If the environment changes, the relative helpfulness of the
variation may as well.
Inheritance
 Inheritance means that the variation/s in question must
have a genotypic basis.


Genetics are the source of variation
Genetic processes can increase the amount of variation in a
population:
 Mutation (#1 source of variation)
 Recombination (crossing-over, independent assortment of
chromosomes in meiosis)
 Sexual reproduction (fertilization in eukaryotes, gene transfer in
prokaryotes)
Inheritance
 Which variations are present in a population = random




Because the genetic processes that produce variation are
random
“Random, rare, and regular”
Not all variations are possible due to genetic/historical
constraints
Variations are not directed by the environment, t.e. they DO
NOT ARISE IN RESPONSE TO “NEED” - common mistake!
Competition
 Resources are limited. All organisms
reproduce as much as possible.
Competition is the inevitable result.

Competition can be interspecific
(between different populations/species)
or intraspecific (between members of
the same population).
 Try to think intraspecific - it’s most
common, most relevant to N.S., and most
overlooked
Differential Reproductive Success
 So everyone’s different…

…because there are so many differences, some of them are
probabilistically bound to happen to be helpful or harmful…
 …you pass your variations on to your kids…
 …and not everyone can have the max number of kids that all
survive.
 Inevitable result: Someone’s going to have more kids,
because they happened to have a variation that gave them
an advantage. (D.R.S.)

And because those kids also get that advantage, and now outrepresent other kids in the next generation, that variation
becomes increasingly common. (N.S.)
Interdisciplinary!
 The development of this concept was
revolutionary… not just in Biology, but
it gave rise to whole other fields as
well!
Political science
 Economics
 Computer science and engineering
(genetic algorithms!)

Random?
 So, is natural selection random?
Nope. Placing a non-random rule
(which variation is best suited to the
present environment) on a random
source (mutation, recombination)
mathematically produces a non-random
result.
 Card game example

Discussion: How NOT To Explain
Evolution by N.S.
 The problem: “A population of giraffes lives on the Serengeti
in East Africa. The best leaves that provide the most nutrition
are at the tops of the trees. In one generation, the average
giraffe’s neck is 1.4 meters long. Fifty generations later, the
average giraffe’s neck is 1.55 meters long. How did this
happen?”
 Explain why each of the following explanations is
wrong, wrong, wrong!
Discussion
 #1: “The more the giraffe stretches its neck to
get to the tops of the trees, the longer its neck
becomes. It passes this longer neck on to its
babies. So over time, necks get longer and
longer.”
Discussion
 #2: “A longer neck is dominant, so over time,
the dominant longer neck allele becomes more
and more common.”
Discussion
 #3: “Longer necks are necessary for the
species to endure, because without them,
giraffes are at risk of starvation. If the giraffe
species doesn’t get a longer neck, then it will
eventually go extinct.”
Discussion
 #4: “Giraffes adapted themselves to their
environment. In their case, because of the tree
height, this meant evolving longer necks.”
Evolutionary Forces
What changes populations?
Forces of evolutionary change
 Natural selection

traits that improve survival
and/or reproduction will accumulate
in the population
 adaptive change
 Genetic drift

frequency of traits can change
in a population due to
chance events
 random change
Natural Selection
 Anything that can reduce reproductive success is termed a
selective force or selective pressure.


But there is no actual “selection” in the typical English
language sense of the word
“Natural selection” - actually kind of a bad term, Darwin later
wished he’d picked another one!
 He used this one because his first chapter was on what was
already known as “artificial selection” and he was setting up a
linguistic contrast
Natural Selection
 Selection acts on any trait that affects
survival or reproduction
predation selection
 physiological selection
 sexual selection

Predation Selection
 Predation selection

act on both predator & prey




behaviors
camouflage & mimicry
speed
defenses (physical & chemical)
Physiological Selection
 Acting on body functions




disease resistance
physiology efficiency (using oxygen, food, water)
biochemical versatility
protection from injury
HOT STUFF!
Some fish had the
variation of producing
anti-freeze protein
5.5 mya
The Antarctic Ocean
freezes over
Effects of Selection
 Changes in the average trait of a population
DIRECTIONAL
SELECTION
STABILIZING
SELECTION
DISRUPTIVE
SELECTION
giraffe neck
horse size
human birth weight
rock pocket mice
Discussion
 It is a fairly common misconception
(bolstered by the narration in countless
nature documentaries!) that natural
selection fashions organisms that are
“perfect,” but this is not the case. Why
not?
Imperfection
 Selection can only act on existing variations.

Which variations are present = chance
 Evolution is limited by historical constraints.

And all organisms bear the scars of their evolutionary
history - more on these in a future week
 Adaptations are often compromises.
 The environment is in constant flux, and natural
selection is a change mechanism, not something that
can predict the future and plan for it!
 Not to mention… what is a “perfect” organism,
anyways?
Evolution is not goal-oriented
An evolutionary trend does not mean that
evolution is goal-oriented.
Evolution can’t
“think ahead!”
Sexual Selection
 Natural selection purely acting on reproductive success, no
involvement of survival
 attractiveness to potential mate
 fertility of gametes
 successful rearing of offspring
Sexual selection
The lion’s mane…
 Females are attracted to males

with larger, dark manes
Correlation with higher
testosterone levels




better nutrition & health
more muscle & aggression
better sperm count / fertility
longer life
 But imposes a cost to male

HOT! Is it worth it??
Sexual selection
 Acts in all sexually
reproducing species
 the traits that get you mates
 sexual dimorphism


influences both morphology & behavior
can act in opposition to traditional
natural selection
Discussion
 Consider these facts:



The difference between female and male gametes is that
female gametes are whole cells, male gametes contain
only genetic material
Sexual selection tends to drive females towards choice
behaviors, and males towards display behaviors
What’s going on?
 In which kinds of species/environments would you expect
to see choosy males and displaying females?
Coevolution
 Two or more species reciprocally
affect each other’s evolution

predator-prey
 disease & host
competitive species
 mutualism

 pollinators & flowers
Discussion
 Coevolution is sometimes compared to a passage
from Lewis Carroll’s Through The Looking Glass, in
which Alice and the Red Queen are trying to run
across a giant chessboard. But because of the
incredible properties of the world through the looking
glass, running as fast as they can only leaves them in
the same spot.
 Why/how do scientists compare this to the
relationship between pathogen and host, or predator
and poisonous prey?
Genetic Drift
 Chance events changing frequency of
traits in a population

not adaptation to environmental conditions
 not selection

founder effect
 small group splinters off & starts a new colony

bottleneck
 some factor (disaster) reduces
population to small number & then
population recovers & expands
again but from a limited gene pool
Founder effect
 When a new population is started
by only a small group of individuals
just by chance some rare alleles may
be at high frequency;
others may be missing
 skew the gene pool of
new population

 human populations that
started from small group
of colonists
 example:
colonization of New World
Distribution of blood types
 Distribution of the O type blood allele in native
populations of the world reflects original settlement
Distribution of blood types
 Distribution of the B type blood allele in native populations of the
world reflects original migration
Out of Africa
Likely migration paths of humans out of Africa
10-20,000ya
10-20,000ya
50,000ya
Many patterns of human traits reflect this migration
Bottleneck effect
 When large population is drastically
reduced by a disaster
famine, natural disaster, loss of habitat…
 loss of variation by chance event

 alleles lost from gene pool
 not due to fitness
 narrows the gene pool
Cheetahs
 All cheetahs share a small number of alleles
less than 1% intraspecies
diversity
 as if all cheetahs are
identical twins

 2 bottlenecks

10,000 years ago
 Ice Age

last 100 years
 poaching & loss of habitat
Measuring
Evolution of Populations
Populations & gene pools
 Concepts
a population is a localized group of
interbreeding individuals. By definition, all
members = same species.
 gene pool is the collection of alleles in the
population

 remember difference between alleles & genes!

allele frequency is how common is that
allele in the population
 how many A vs. a in whole population
Evolution of populations
 Evolution = change in allele frequencies
in a population


hypothetical: what conditions would
cause allele frequencies to not change?
non-evolving population
REMOVE all agents of evolutionary change
1. very large population size (no genetic drift)
2. no migration (no gene flow in or out)
3. no mutation (no genetic change)
4. random mating (no sexual selection)
5. no natural selection (everyone is equally fit)
Hardy-Weinberg equilibrium
 Hypothetical, non-evolving population

preserves allele frequencies
 Serves as a model (null hypothesis)


natural populations rarely in H-W equilibrium
useful model to measure if forces are acting on a
population
 measuring evolutionary change
G.H. Hardy
mathematician
W. Weinberg
physician
Hardy-Weinberg theorem
 Allele frequencies
assume 2 alleles = B, b
 frequency of dominant allele (B) = p
 frequency of recessive allele (b) = q

 frequencies must add to 1 (100%), so:
p+q=1
BB
Bb
bb
Hardy-Weinberg theorem
 Counting Individuals



frequency of homozygous dominant: p x p = p2
frequency of homozygous recessive: q x q = q2
frequency of heterozygotes: (p x q) + (q x p) = 2pq
 frequencies of all individuals must add to 1 (100%), so:
p2 + 2pq + q2 = 1
BB
Bb
bb
H-W formulas
 Alleles:
p+q=1
B
 Individuals:
p2 + 2pq + q2 = 1
BB
BB
b
Bb
Bb
bb
bb
Discussion
population:
100 cats
84 black, 16 white
How many of each
genotype?
p2=.36
BB
q2 (bb): 16/100 = .16
q (b): √.16 = 0.4
p (B): 1 - 0.4 = 0.6
2pq=.48
Bb
q2=.16
bb
What
are thepopulation
genotype is
frequencies?
Must
assume
in H-W equilibrium!
Using Hardy-Weinberg equation
p2=.36
Assuming
H-W equilibrium
2pq=.48
q2=.16
BB
Bb
bb
p2=.20
=.74
BB
2pq=.64
2pq=.10
Bb
q2=.16
bb
Null hypothesis
Sampled data
How do you
explain the data?
Discussion
 Populations can be in Hardy-Weinberg

equilibrium at one locus (gene), but out
of equilibrium at another.
What could cause that to be the case?
Discussion
 Practice problems, everybody’s
favorite!
Discussion
 An example AP-style question:
 “PKU is a rare autosomal recessive disorder. Babies


born with PKU do not make the liver enzyme
necessary to metabolize the amino acid
phenylalanine to the amino acid tyrosine. 1 in every
10,000 babies (a frequency of 0.0001) is born with the
disease, and when they grow older, should avoid
some foods.
Assume that the mutation rate in the PKU gene is so
low as to be negligible, and that mates are not
chosen on the basis of the manufacture of this liver
enzyme. Further assume that diet modifications are
available to all humans at no particular cost,
rendering selective force on this locus negligible.
What percent of the population would you expect to
be carriers of the PKU disease allele?”
Application of H-W principle
 Sickle cell anemia

inherit a mutation in gene coding for
hemoglobin
 oxygen-carrying blood protein
 recessive allele = HsHs
 normal allele = Hb

low oxygen levels causes
RBC to sickle
 breakdown of RBC
 clogging small blood vessels
 damage to organs

often lethal
Sickle cell frequency
 High frequency of heterozygotes
1 in 5 in Central Africans = HbHs
 unusual for allele with severe
detrimental effects in homozygotes

 1 in 100 = HsHs
 usually die before reproductive age
Why is the Hs allele maintained at such high
levels in African populations?
Suggests some selective advantage of
being heterozygous…
Single-celled eukaryote parasite
(Plasmodium) spends part of its
life cycle in red blood cells
Malaria
1
2
3
Heterozygote Advantage
 In tropical Africa, where malaria is common:

homozygous dominant
 die or reduced reproduction from malaria: HbHb

homozygous recessive
 die or reduced reproduction from sickle cell anemia: HsHs

heterozygote carriers are relatively free of both: HbHs
 survive & reproduce more, more common in population
Hypothesis:
In malaria-infected
cells, the O2 level is
lowered enough to
cause sickling which
kills the cell & destroys
the parasite.
Frequency of sickle cell allele
& distribution of malaria
Discussion
 How does a new species arise?
First…what is a species?
 Biological species concept



defined by Ernst Mayr
population whose members can interbreed & produce
viable, fertile offspring
reproductively compatible
Distinct species:
songs & behaviors are different
enough to prevent interbreeding
Eastern Meadowlark Western Meadowlark
But that doesn’t capture every
situation
 Consider
Ensatina
salamanders
. How many
species?
Which ones
are different
species?
Species Definitions
 Other definitions include:



Morphological or typological - They conform to
the same body plan.
Phylogenetic or evolutionary - Share a
common ancestor and a unique evolutionary
history.
Ecological - Share a specific niche, unique to
them and them alone.
 “Species” is a human language box. Never
forget that nature exists on a continuum!
Discussion
 Which definitions work or don’t work to
determine whether or not you’re
examining different species if you’re
studying…
Bacteria in a lab petri dish?
 Hooved mammals in the modern-day
arctic?
 Dinosaurs?
 Ancient algae?

How and why do new species originate?
 Species are created by a series of evolutionary
processes

populations become isolated - no gene flow between
them
 geographically isolated and/or
 reproductively isolated

isolated populations
evolve independently
 Isolation

allopatric
 geographic separation

sympatric
 still live in same area
PRE-zygotic barriers
 An obstacle to mating or fertilization
geographic isolation
behavioral isolation
ecological isolation
temporal isolation
mechanical isolation
gametic isolation
Ammospermophilus spp
Geographic isolation
 Species occur in different areas
physical barrier
 allopatric speciation

 “other country”
Harris’s antelope
squirrel inhabits
the canyon’s
south rim (L). Just
a few miles away
on the north rim
(R) lives the
closely related
white-tailed
antelope squirrel
Ecological isolation
 Species occur in same region, but occupy different
habitats so rarely encounter each other

reproductively isolated
2 species of garter snake, Thamnophis,
occur in same area, but one lives in water &
other is terrestrial
lions & tigers could
hybridize, but they
live in different
habitats:
 lions in grasslands
 tigers in rainforest
Temporal isolation
 Species that breed during different times of day,
different seasons, or different years cannot mix
gametes


reproductive isolation
sympatric speciation
 “same country”
Eastern spotted skunk
(L) & western spotted
skunk (R) overlap in
range but eastern mates
in late winter & western
mates in late summer
sympatric speciation?
Behavioral isolation
 Unique behavioral patterns & rituals isolate species


identifies members of species
attract mates of same species •
 courtship rituals, mating calls
 reproductive isolation
Blue footed boobies mate
only after a courtship display
unique to their species
Recognizing your
own species
courtship songs of sympatric
species of lacewings
courtship display of
Gray-Crowned Cranes, Kenya
firefly courtship displays
sympatric speciation?
Mechanical isolation
 Morphological differences can prevent successful
mating

reproductive isolation
Plants
Even in closely related
species of plants, the
flowers often have distinct
appearances that attract
different pollinators.
These 2 species of monkey
flower differ greatly in
shape & color, therefore
cross-pollination does not
happen.
Mechanical isolation
Animals
 For many insects, male &
female sex organs of
closely related species do
not fit together, preventing
sperm transfer

lack of “fit” between sexual organs:
hard to imagine for us… but a big issue for insects with
different shaped genitals!
Damsel fly penises
sympatric speciation?
Gametic isolation
 Sperm of one species may not be able to fertilize eggs of
another species

mechanisms
 biochemical barrier so sperm cannot penetrate egg
 receptor recognition: lock & key between egg & sperm
 chemical incompatibility
 sperm cannot survive in female reproductive tract
Sea urchins release sperm
& eggs into surrounding
waters where they fuse &
form zygotes. Gametes of
different species— red &
purple —are unable to fuse.
POST-zygotic barriers
 Prevent hybrid offspring from
developing into a viable, fertile adult
reduced hybrid viability
 reduced hybrid fertility
 hybrid breakdown

zebroid
sympatric speciation?
Reduced hybrid viability
 Genes of different parent species may
interact & impair the hybrid’s development
Species of salamander
genus, Ensatina, may
interbreed, but most
hybrids do not complete
development & those
that do are frail.
Reduced hybrid fertility
 Even if hybrids are vigorous
they may be sterile

chromosomes of parents may differ in number
or structure & meiosis in hybrids may fail to
produce normal gametes
Mules are vigorous,
but sterile
Horses have 64
chromosomes
(32 pairs)
Donkeys have 62
chromosomes
Mules have 63 chromosomes! (31 pairs)
sympatric speciation?
Hybrid breakdown
 Hybrids may be fertile & viable in first
generation, but when they mate offspring
are feeble or sterile
In strains of cultivated rice,
hybrids are vigorous but
plants in next generation are
small & sterile.
On path to separate species.
Rate of Speciation
 When considering speciation events over
geological time: Does speciation happen
gradually or rapidly, uniformly or
unevenly?

Gradualism
 Charles Darwin
 Charles Lyell

Punctuated equilibrium
 Stephen Jay Gould
 Niles Eldredge
Niles Eldredge
Curator
American Museum of Natural History
Gradualism
 Gradual divergence over
long spans of time


assume that big changes
occur as the accumulation
of many small ones
events can increase or
decrease speciations
worldwide, but overall
speciation proceeds fairly
regularly
Punctuated Equilibrium
 Rate of speciation is not
constant



Organisms are in
“stasis” for much of
their history, with little or
no change
When speciation occurs,
it tends to be in a rapid
burst
Species undergo rapid
change when they 1st
bud from parent
population
Time
Discussion
 Based upon what you know of
evolutionary history, where do you fall
in the debate?
Speciation Rates
 Regardless of whether punctuated
equilibrium or gradualism holds,
speciation rates vary by species and
circumstance
Speciation can occur over a scale of
millions of years, or much more rapidly!
 Polyploidy in plants increases
speciation rate to, in some cases, only a
few years

Polyploidy and Hybrid Speciation
 Unlike in animals, in plants, duplicating
the genome (polyploidy) isn’t fatal.
 The pollination reproduction method
means plants hybridize more often and
more readily than animals on average
Sometimes, a diploid hybrid is
sterile, but a triploid or tetraploid
hybrid isn’t.
So if two plants hybridize, and the
sterile offspring undergoes
polyploidy, it can produce fertile,
speciated offspring!
Polyploidy and Hybrid Speciation
 This has been observed in species like
the Evening Primrose,
Raphanobrassica, Hemp Nettle, and the
Maidenhair Fern.
Speciation Rates
 In all species, when a new habitat or
new niche becomes available,
speciation rates tend to increase
 Adaptive radiation - ecological &
phenotypic diversity in a rapidly
multiplying lineage
Discussion
 Scientists generally break it down into
two main reasons why this causes a
burst in speciation events. What do
you think they could be?
Adaptive Radiation
 Innovation - The evolution of one trait can make the
evolution of others possible

Ex: A harder shell on eggs not only made it possible to
lay them out of water, it opened up many new avenues
for expansion, which drove further diversification
 Opportunity - Unoccupied niches, such as on new
land or after a mass extinction, which new occupiers
adapt to
Speciation Rates
 Ex: Darwin’s finches
 Ex: An explosion in bivalve species
diversity after the loss of brachiopods
in the “Great Dying,” or Permian
extinction 250 mya
Extinction
 But, of course, extinction rates also
fluctuate

Higher in times of environmental stress
% of families
extinct
Million years ago
Discussion
 A population’s ability to respond to
environmental changes is dictated, in
part, by its level of genetic diversity.
 Which do you think is most resistant to
extinction and why: high-diversity or
low-diversity?